Add test / fix static analysis error

Author: z4kn4fein

src/Traits/Copyable.php

@@ -34,7 +34,11 @@ public function copy(
             $major == null ? $this->major : $major,
             $minor == null ? $this->minor : $minor,
             $patch == null ? $this->patch : $patch,
-            $preRelease === null ? $this->preRelease : $preRelease,
+            $preRelease === null
+                ? $this->preRelease === null
+                    ? null
+                    : (string)$this->preRelease
+                : $preRelease,
             $buildMeta === null ? $this->buildMeta : $buildMeta
         );
     }

tests/CreateTest.php

@@ -44,6 +44,19 @@ public function testCopy()
         $this->assertEquals("2.3.4-alpha+build", (string)$version->copy(2, 3, 4));
     }
 
+    public function testCopyStable()
+    {
+        $version = Version::create(0, 0,0);
+        $this->assertEquals($version, $version->copy());
+
+        $this->assertEquals("2.0.0", (string)$version->copy(2));
+        $this->assertEquals("0.2.0", (string)$version->copy(null, 2));
+        $this->assertEquals("0.0.2", (string)$version->copy(null, null, 2));
+        $this->assertEquals("0.0.0-beta", (string)$version->copy(null, null, null, "beta"));
+        $this->assertEquals("0.0.0+build2", (string)$version->copy(null, null, null, null, "build2"));
+        $this->assertEquals("2.3.4", (string)$version->copy(2, 3, 4));
+    }
+
     public function testSuffixes()
     {
         $version = Version::parse("0.1.2-alpha+build");